Fluorescent lamp with silicon dioxide coating

ABSTRACT

An improved low pressure mercury vapor discharge lamp, particularly of the fluorescent type. The inner wall of the glass envelope is coated with a layer of SiO 2  particles which, in turn, is coated with a phosphor layer. The SiO 2  particles have a particle size of below about 100 nm. The SiO 2  particle containing layer containing between 0.5 and 0.7 mg of SiO 2  particles per square centimeter of glass envelope which is coated.

BACKGROUND OF THE INVENTION

The invention provides an improved low pressure mercury vapor dischargelamp, in particular a fluorescent lamp, comprising a glass envelope withan inner wall phosphor coating and with a silicon dioxide (SiO₂) coatingbetween the phosphor coating and the envelope.

It is known to apply to the inner wall of the lamp envelope acontinuous, three-dimensional film of a structure of bonded silicon andoxygen atoms (NL-PA 68 13 725). It is a homogeneous coating of athickness of preferably 0.1 `to 0.4μ. The coating is intended to preventa reaction between the mercury in the lamp and the alkaline componentscontained in the glass wall which, if the reaction occurred, wouldresult in the production of amalgam and cause a blackening of theenvelope, and thus an accelerated reduction of the light out-put andshortening of the lamp life.

It is the subject of the invention to improve the lamp with respect toluminous efficacy, luminous flux and production costs by means of asilicon dioxide intermediate coating.

THE INVENTION

The low pressure mercury vapor discharge lamp, in particular afluorescent lamp, comprising a glass envelope with a phosphor coating onthe inner wall and with a coating of silicon dioxide (SiO₂) between thephosphor coating and the envelope, is characterized in that the SiO₂-coating is granular and has a thickness of between 0.05 and 0.7 mg/cm².The particle size of the SiO₂ granules is below 100 nm.

The amalgam produced in prior art lamps resulting from reaction ofmercury with the alkaline components of the glass is prevented in lampsaccording to the present invention by means of a SiO₂ -coating. Inaddition to the dense glass wall coating having hardly any porosity, andthe thus effected screening against the mercury atoms, surprisingly, anoptical effect of the coating results from suitable dimensioning of theSiO₂ -coating. With a coating thickness according to the invention ofbetween 0.05 and 0.7 mg/cm², preferably between 0.08 and 0.4 mg/cm², anda particle size smaller than 100 nm, there are 10¹² to 10¹⁵ diffusioncenters per cm², the diameter of which lies below the wavelength ofvisible light, and also clearly below the wavelength of UV-radiationproduced by the discharge. Coatings of a thickness between 0.15 and 0.2mg/cm² produce particularly good results.

Although the packing density of the diffusion centers is very high, theluminance behavior of this coating can approximately be described by theRaleigh scatter. The portion of the luminance radiation thereforechanges with .sup.λ 1/4, i.e., with the 4th power of the wavelength; theluminance increasing as the wavelength of the incident radiation becomessmaller. This effect is very advantageous because the mercury dischargecontains, in addition to 254 nm radiation, a considerable portion(approximately 10% of the UV-radiation) of 185 nm radiation, and thusthis shortwave portion is also to a great extent reflected into thephosphor coating and cannot penetrate to the envelope wall. In the caseof lamps without luminance coating, approximately 30% to 50% of the 185nm radiation is destroyed at the glass wall since, in general, thisradiation is only poorly absorbed by the phosphor coating.

In particular, lamps of reduced diameter (preferably of 26 mm diameter)show the advantageous effect of the SiO₂ coating, since with these lampsthe UV-radiation density increases by approximately 30% at the locationof the phosphor material and at the glass wall due to the higher currentdensity and the reduced area covered with phosphor material.

The combination of a SiO₂ -coating with a three band phosphor materialas applied in lamps known under the trademark Lumilux is of particularadvantage.

The application of the SiO₂ -coating in the thickness according to thisinvention also permits a reduction of the amount of phosphor materialwhich is needed. This is due to the great diffusion capacity of the SiO₂-coating according to the invention in the UV-range, causing a portionof the luminance UV-radiation to be directed back into the phosphormaterial.

The increased utilization of the UV-radiation when a SiO₂ -coating of athickness of 0.05 to 0.7 mg/cm² is on the glass envelope also results inhigher luminous efficacies. With thinner coatings, it is impossible toobtain the necessary number of diffusion centers. With thicker coatings,an observable amount of absorption of the visible light occurs, causinga reduction in the luminous efficacy.

THE DRAWINGS

The invention, which can be utilized in all fluorescent lamps, isillustrated by means of the exemplified embodiments in FIGS. 1, 2, 3, 4and 5.

FIG. 1 depicts a plan view of a lamp;

FIG. 2 depicts a cross-section of the lamp;

FIG. 3 is a graph of luminance as a function dependent upon thewavelength λ;

FIG. 4 is a graph of the luminous efficacy η_(L) as a function dependentupon the operating time t in hours h; and

FIG. 5 shows the luminous efficacy η_(L) as a function dependent on theweight of the coating of the phosphor material mg/cm² in %.

The lamp depicted in FIG. 1 comprises a glass envelope 1 of a diameterof preferably 26 mm, each end 2 of said envelope being provided with asealed-in electrode 3, 4. A coating 5 of highly dispersed SiO₂ particleshaving an approximate thickness of 0.18 mg/cm² is applied to the innerwall of the envelope 1 (FIG. 2), said coating consisting of 40 to 70layers of particles of a diameter smaller than 100 nm. This coating, inturn, is covered by the usual phosphor coating 6 of, e.g.,halophosphates, three band phosphor materials, among others. Beforeapplying the phosphor coating, the inner surface of the envelope iswetted with a suspension of SiO₂ -powder, binder and solvent.Nitrocellulose has proven to be suitable as binder and butylacetate assolvent, or polymethacrylate as binder and water as solvent.

FIG. 3 reports that the luminance for the SiO₂ coating according to theinvention is approximately 50% for the 185 nm radiation andapproximately 30% for the 254 nm radiation. FIG. 4 reports the effect ofthe SiO₂ -coating (curve a) with respect to the luminous efficacy η_(L)in lm/W. Curve b reports the corresponding data for a lamp without theSiO₂ -coating. After 5,000 hours of operation, the luminous efficacyreported by curve a increases to approximately 10% with respect to curveb.

In FIG. 5, the curve a reports data for a lamp with a SiO₂ -coatingaccording to this invention, and a curve b for a lamp without thiscoating. FIG. 5 shows clearly that the maximum of the luminous efficacyη_(L) in lm/W shifts toward lower phosphor coating weight. About 10% ofthe phosphor material can be saved by the application of the SiO₂-coating according to the invention, with the added benefit that theluminous efficacy is increased when compared to the usual fluorescentlamp types that do not have the coating according to this invention.When improved luminous efficacy is not sought, up to 20% of the phosphormaterial can be saved per lamp.

The designation of the thickness of the SiO₂ particle containing coatingon the inside of the glass envelope is in terms of the weight of SiO₂particles in said coating layer per square centimeter of the glassenvelope which is coated.

The SiO₂ coating is applied by wetting the inner surface of the envelopewith a suspension of the SiO₂ particles in the polymeric binder whichalso contains solvent. The relative proportion by weight of SiO₂particles to binder is between 100:1 and 100:35, and preferably between100:3 and 100:20. The content of solid material (SiO₂) in the paste isbetween about 0.2 and 8 percent by weight of the coating composition andpreferably between 0.8 and 5 percent by weight.

The lower limit for the SiO₂ particle size is at about 2 nm. The primaryparticles have preferably a size of from 7 to 20 nm, the agglomeratespreferably of from 10 to 70 nm. As a three band phosphor material issuited a known phosphor which consists of europium-activated yttriumoxide, terbium-activated cerium magnesium aluminate andeuropium-activated barium magnesium aluminate.

What is claimed is:
 1. An improved low pressure mercury vapor dischargelamp, comprising a glass envelope having an electrode at each end withan inner wall phosphor coating substantially completely covering acoating of silicon dioxide SiO₂ which is between the phosphor coatingand the envelope,the improvement comprising the SiO₂ in said coatingbeing in the form of SiO₂ particles having a particle size between about2 and 100 nm and said SiO₂ particles containing coating being in anamount between 0.05 and 0.7 mg of SiO₂ particles in said coating persquare centimeter of glass envelope which is coated.
 2. The low pressuremercury vapor discharge lamp of claim 1, wherein said SiO₂particle-containing coating is in an amount between 0.08 and 0.4 mg/cm².3. The low pressure mercury vapor discharge lamp of claim 1, whereinsaid SiO₂ particle-containing coating a thickness between 0.15 and 0.2mg/cm².
 4. A method of manufacturing a low pressure mercury vapordischarge lamp according to claim 1 or 2 or 3, characterized in thatsaid SiO₂ particle-containing coating is formed on the inner surface ofthe glass envelope by wetting said inner surface with said amount ofSiO₂ coating in the form of a suspension of SiO₂ -powder in binder andsolvent and then applying the phosphor coating.
 5. The method of claim4, wherein said SiO₂ is present in the binder in a weight ratio or SiO₂: binder between 100:1 and 100:35.
 6. The method of claim 4, whereinsaid SiO₂ is present in the binder in a weight ratio of SiO₂ : binderbetween 100:3 and 100:20.
 7. The low pressure mercury vapor dischargelamp of any one of claims 1, 2 or 3, wherein the particle size of saidSiO₂ particles is between 7 and 20 nm.
 8. The low pressure mercury vapordischarge lamp of claim 7, wherein said particles of SiO₂ formagglomerates of the size between 10 and 70 nm.